14 MOLLUSCA FROM THE CRAG. 



OsTREA DENTICULATA. Besh. 2d ed. Lfira., torn, vii, p. 225. 

 — Broc. Conch. Foss. Subap., p. 5GH, 1814. 



— FOLIOSA. Broc. Conch. Foss. Subap., p. 56.3, 1814. 



— UNGULATA. Nijst. Coq. Foss. dc Belg., p. .325, pi. 24, fig. S ; pi. 2C, fig. 8 ; and 



pi. 34, fig. 1,«, a, b, 1844. 



— BOEEALis ? Desk. 2d ed. Lara., torn, vii, p. 220. 



— — Gould. Inv. Massach., p. 137, 1841. 



— SPECTRUM. {Leathes, M. S.) Woodward Synop. Tab. Brit. Org. Rem., p. 20, 1830. 



— — S. Wood. Catalogue, 1840. 



— LAMELLOSA. Gold/. Pet. Germ., vol. ii, p 18, t. 78, fig. 3, a, b. 

 OsTREUM VULGARE. Bacostu. Ikit. Conch., p. 154, pi. 11, fig. 6, 1778. 

 AvRicuLARiA MAXIMA. Dale. Hist, and Antiq. of Harwich, p. 291, t. xii, fig. 2, 1730. 



Ency. Method., pi. 183, figs. 3, 4 ? pi. 184, figs. 7, 8. 



Sj)ec. Char. Testa variabile, pleriimque ovato-rotimdatd, basi sitbattemiatd, coticen- 

 trice lamellosd, interdum radiato-costatd, costis imbricatis, undtilatis ; valvd superiori 

 plana. 



Shell variable, for the most part of a roundedly ovate form ; base of the shell, 

 slightly attenuated, concentrically lamcllated, sometimes costated with imbricated and 

 undulated ribs ; upper valve flat. 

 Diameter, 3 inches. 



Localilij. Cor. Crag, Ramsholt, Sudbourn, Gedgravc. 

 Red Crag, Passim. 



Recent, Britain. North America .- Mediterranean ? 

 The earliest appearance of the true edible Oyster seems to have been in the 

 Coralline Crag Period, so far as it is possible to determine a species in this truly 

 variable genus, and its diversified forms were tlien as great as we see them in the 

 recent shell of the present seas. In the same deposit, at Ramsholt, and in the same 

 bed in close proximity, are two very distinct varieties, from which, as well as from a 

 great dissimilarity of form, solidity of shell, and other differences, it was presumed at 

 the time my Catalogue was drawn up, that they were specifically distinct ; but the 

 variety then considered different, and which passed under the provisional nanu- of 

 apectrum, corresponds witli the recent shell that is now determined by British Con- 

 chologists to be only a local variation, and tlicy are, therefore, here united into one 

 species. At Ramsholt there is a complete bed of this shell (parasitica), sometimes 

 attached to each other in clusters, or often to the large species of Balainis, so abun- 

 dant in that locality ; at this place, also, the thick and ponderous variety (fig. 1, a), is 

 occasionally met with, but not in any great profusion ; tiic latter shell is at this place 

 more isolated in its habits and regular in form, and is sometimes marked with nearly 

 obsolete radiating costee, with a very rugose exterior to the lower or adherent valve, 

 while in the upper valve there are no markings, except the regular lines of growth ; 

 in the other variety, tlierc is less appearance of the radiating ribs with a more 

 lamellated exterior, and the shell is much thinner, and less regular in shape, partaking 



